The present invention relates to an electric lamp with high efficiency, good color rendering, and high lamp lumen maintenance.
Discharge lamps produce light by ionizing a vapor fill material such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficacies.
Conventionally, the discharge chamber in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into desired chamber geometries after being heated to a softened state. Fused quartz, however, has certain disadvantages which arise from its reactive properties at high operating temperatures. For example, in a quartz lamp, at temperatures greater than about 950-1000° C., the halide filling reacts with the glass to produce silicates and silicon halide, which results in depletion of the fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall, which causes depletion of the fill. Both depletions cause color shift over time, which reduces the useful lifetime of the lamp. Color rendition, as measured by the color rendering index (CRI or Ra) tends to be moderate in existing quartz metal halide (QMH) lamps, typically in the range of 65-70 CRI, with moderate lumen maintenance, typically 65-70%, and moderate to high efficacies of 100-150 lumens per watt (LPW). U.S. Pat. Nos. 3,786,297 and 3,798,487 disclose quartz lamps which use high concentrations of cerium iodide in the fill to achieve relatively high efficiencies of 130 LPW at the expense of the CRI. These lamps are limited in performance by the maximum wall temperature achievable in the quartz arctube.
A conventional metal halide lamp is fabricated by charging, in a light-transmitting quartz tube, mercury, an inert gas, e.g., argon, and a halide mixture including at least one kind of rare earth halide and an alkali metal halide, and sealing the tube.
Ceramic discharge chambers were developed to operate at higher temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. In general, CMH lamps are operated on an AC voltage supply source with a frequency of 50 or 60 Hz, if operated on an electromagnetic ballast, or higher if operated on an electronic ballast. The discharge is extinguished, and subsequently re-ignited in the lamp, upon each polarity change in the supply voltage.
U.S. Pat. No. 6,583,563 discloses a ceramic metal halide lampcapable of operating at over 150 watts. The body portion has a length of an inner diameter of about 9.5 mm and outer diameter of about 11.5 mm. U.S. Pat. No. 6,555,962 discloses a metal halide lamp with a power rating of 200 W or more to be used with an existing ballast for a high pressure sodium (HPS) lamp of like power rating. The inside diameter D and inside length L are selected so as to provide an aspect ratio L/D of between 3 and 5. U.S. application Ser. No. 10/792,996, filed Mar. 4, 2004, discloses a CMH lamp having a ceramic arctube in which the length and diameter are selected such that the lamp is capable of operating in the range of 250-400 W with a CRI of at least 85 and an efficiency of at least 90 lumens/watt.
For commercial metal halide lamps of high wattage, lumen maintenance (measured as the percentage of lumens retained at the mean lifetime of the lamp as compared with the lumens at 100 hours) is generally low, typically only about 65% or less, often only about 50%. Thus, a conventional 400 W lamp, while it may have a high initial lumen output, will only have a lumen output comparable to a new 250 W lamp by its mean lifetime of about 8000-10,000 hours.
The present invention provides a new and improved metal halide lamp capable of operating at high or low power which has a high efficiency and good lamp lumen maintenance.